An abstraction that represents the shared resources of a persistence engine.
This could include file handles, db connection pools, caches, etc.
For example for BDB it holds the various environments, for jdbc it holds a
connection pool reference
This should be called StorageEngineFactory but that would leave us with the
indignity of having a StorageEngineFactoryFactory to handle the mapping of
store type => factory. And we can't have that.